If you want to tell whether someone is conservative or liberal, what are a couple of completely nonpolitical questions that will give a good clue?

How’s this: Would you be willing to slap your father in the face, with his permission, as part of a comedy skit?

And, second: Does it disgust you to touch the faucet in a public restroom?

Studies suggest that conservatives are more often distressed by actions that seem disrespectful of authority, such as slapping Dad. Liberals don’t worry as long as Dad has given permission.

Likewise, conservatives are more likely than liberals to sense contamination or perceive disgust. People who would be disgusted to find that they had accidentally sipped from an acquaintance’s drink are more likely to identify as conservatives.

The upshot is that liberals and conservatives don’t just think differently, they also feel differently. This may even be a result, in part, of divergent neural responses.

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The larger point is that liberals and conservatives often form judgments through flash intuitions that aren’t a result of a deliberative process. The crucial part of the brain for these judgments is the medial prefrontal cortex, which has more to do with moralizing than with rationality. If you damage your prefrontal cortex, your I.Q. may be unaffected, but you’ll have trouble harrumphing.

One of the main divides between left and right is the dependence on different moral values. For liberals, morality derives mostly from fairness and prevention of harm. For conservatives, morality also involves upholding authority and loyalty — and revulsion at disgust.

Some evolutionary psychologists believe that disgust emerged as a protective mechanism against health risks, like feces, spoiled food or corpses. Later, many societies came to apply the same emotion to social “threats.” Humans appear to be the only species that registers disgust, which is why a dog will wag its tail in puzzlement when its horrified owner yanks it back from eating excrement.

Psychologists have developed a “disgust scale” based on how queasy people would be in 27 situations, such as stepping barefoot on an earthworm or smelling urine in a tunnel. Conservatives systematically register more disgust than liberals. . . .

It appears that we start with moral intuitions that our brains then find evidence to support. For example, one experiment involved hypnotizing subjects to expect a flash of disgust at the word “take.” They were then told about Dan, a student council president who “tries to take topics that appeal to both professors and students.”

The research subjects felt disgust but couldn’t find any good reason for it. So, in some cases, they concocted their own reasons, such as: “Dan is a popularity-seeking snob.”